With some 700 species and at least 10,000 hybrids to choose from, it is little wonder the rhododendron has claimed the affection and interest of gardeners.
Whenever a breathtaking specimen is seen, the desire to own one like it becomes overwhelming.
Before success with rooted cuttings, rhododendrons were propagated either by grafting or layering. Unfortunately, seedlings do not come true.
Rooted cuttings, however, will resemble the parent since they are a piece of it brought into independent existence.
Perfecting Rooted Rhododendron Cuttings
Rooting rhododendron cuttings were first perfected by G. G. Nearing of Ramsey, N.J., in the 1920s. To this day, the Nearing propagating frame is still a dependable method.
Though cuttings can be rooted at almost any time of the year, most propagators take cuttings in September and October, selecting terminal shoots produced during the preceding three-month period.
Any terminal shoot 4″ to 6″ inches in length can be rooted. However, the best plants develop from slender cuttings with leaf buds and no flower buds.
These shoots are found in the middle or lower portion of larger plants. If only flower-bud bearing shoots are available, remove the buds after cutting.
Cuttings and Rooting
Cuttings are best taken in the morning when the turgor is highest and should be placed immediately in a polyethylene bag, then closed and carefully labeled.
Cuttings may be kept sealed in a cool shaded spot for as long as two weeks, though it is not advised.
(Cuttings have been shipped to me from Japan in this manner arriving three weeks later, and have rooted well.)
Remove all leaves from the cutting except four or five at the top. Shorten the crown leaves by cutting off 50% of their length if the leaf exceeds 4″ in length.
Many propagators frown on this last step, believing the plant may be injured and rooting impaired. However, experiments show a cutting thus prepared roots just as well and continues to vigorous growth.
Moreover, this practice permits more plants to occupy the same space and allows more light to reach all leaves.
The cuttings are now ready to dip in a solution of fungicide and DDT to kill any larvae or fungi which are sometimes found on the leaf under the surface. Then bruise the stem 1-1 ½” at the cut end.
Two such bruises on opposite sides give the best results. Use a sharp knife, scrape gently, and avoid deep cuts.
Removing the surface layer of cells leaves a raw, wet surface which will expedite water and hormone absorption. This done, dip the cutting in a root-inducing hormone and insert it into the bed.
Controlled experiments leave little doubt about the value of hormones in rooting cuttings. Moreover, hormones are now available in liquid form or suspended in talc in weak dilutions, either singly or in groups.
The freshly bruised cutting is dipped in either medium, the excess shaken off, and the cutting inserted into the bed, burying half the stem. Then thoroughly water and cover it with a polyethylene tent.
Various Rooting Medium
The rooting medium used varies. Sand, sphagnum moss, styrofoam, vermiculite, perlite, and peat moss, either alone or in various combinations, are used.
Use a mixture of German peat moss (because of its coarseness and low pH) and coarse industrial perlite.
The peat moss is thoroughly broken up by hand, mixed dry with the perlite, and packed firmly in a large flat, 4″ inches deep.
Elective Width and Flat
The width and length of the flat are elective. There must be ½” spaces between the bottom slats to ensure drainage.
A word of caution regarding wetting the medium. Peat is stubbornly water-repellent.
It is necessary to water the flat repeatedly for several days before inserting the cuttings. The last watering is done with a watering can containing a heaping tablespoon of fungicide and an equal amount of wettable 10% DDT dust for every two gallons of water.
This will sterilize the bed. The cuttings are then inserted, allowing 2 ½” between each cutting and each row’. A final watering will firm them into the peat-perlite mixture.
Cover the flat with the thinnest polyethylene available, leaving at least a 10″ space for air above the leaves.
Polyethylene will permit gasses such as oxygen and nitrogen to pass through but will prevent moisture loss.
Sweat Box With Humidity
We now have a “sweat box” with almost 100% humidity. This is necessary since the cutting is a live organism that has been severed from the parent root system.
Water transpiration constantly occurs through the tiny stomas of the leaves, but high humidity in the polyethylene tent prevents the loss of plant fluids.
Indeed, water may be absorbed by the foliage from the moisture-laden air pocket if the cutting is deficient in water. Within a few hours, beads of water will accumulate on the inside of the tent and will be seen for weeks to come.
These droplets must always be present, for watering will not be necessary as long as moisture appears on the film.
Propagating Unit
The propagating unit should be kept between 68 and 72° degrees Fahrenheit. Give it as much light as possible but no direct sunlight.
Polyethylene traps infrared heat waves from sunlight, and the air temperature will rise dangerously and burn the foliage should the direct sun strike the tent for even a short period.
In eight to ten weeks, roots should be well developed. Dwarf hybrids root frequently in six weeks. When well rooted, the cuttings will resist gentle tugging.
Each plant, with its roots embedded in a ball of peat, is lifted by gently tearing it out of bed or by cutting out a square surrounding the stem and transplanting it to another flat containing equal parts of sand, peat moss, and leaf mold.
The plastic tent is no longer necessary. Heat should be continued for two to three more weeks. Now, the plants may be exposed to sunshine if cheesecloth netting provides partial shade.
Discontinue the heat at the end of three weeks and place the flat in a cold frame to harden the plants and keep them dormant until spring.
Transplant to a bed rich with peat and leaf mold or old manure when frost is out of the ground. Lightly feeding acid azalea fertilizer in early summer will yield a lush second growth.
44659 by Betty Brinhart